Clean and wash the cauliflower. Cut a few florets from the stalk and slice thinly. Cover and put aside. Chop the rest of the cauliflower roughly. Peel and chop the onions.
Heat 2 tablespoons of butter in a large saucepan. Brown the onions in it. Add chopped cauliflower and steam for about 5 minutes. Dust with flour and sauté briefly. 3⁄4 l Pour on water and cream and bring to the boil.
Stir in the stock and simmer the soup for about 15 minutes, stirring occasionally.
In the meantime, roughly chop the pistachios. Cut cress from the bed. Heat 2 tablespoons of butter in a large pan. Fry the cauliflower slices for 3-5 minutes, turning them over. Season with salt.
Finely puree the soup with a hand blender. Season to taste with salt, pepper, nutmeg and lemon juice. Serve the soup. Spread cauliflower slices, cress and pistachios on top.